“This kind of attack requires that the attacker be able to monitor all traffic of targeted users and needs the CPU resources to process all the data,” DIT said.

Web users in the country - which tightly restricts Internet access - had trouble accessing numerous sites on Tuesday, said Greatfire.org, which tracks the vast Chinese online censorship apparatus known as the Great Firewall.

“We have conclusive evidence that this outage was caused by the Great Firewall,” it said on its website, calling the incident “one of the largest Internet outages ever in China”.

The state news agency Xinhua raised the possibility of hacking, and the official China Internet Network Information Centre attributed the breakdown to a “root server for top-level domain names”.

But Greatfire.org cast doubt on those claims, citing technical tests and saying such an act was “not enough to cause this outage.” China has about a half billion Internet users.

China's vast censorship apparatus proactively suppresses any information or websites online deemed sensitive, from popular sites such as Facebook and Twitter to a frequently updated list of search terms. - Sapa-AFP